It can also be easily checked that Eq. (REF ) reduces to
the standard Langevin equation in
WI [1]}, [2]} for \(c_{s}=1\) , i.e.,
\(\epsilon _{s}=0\) . As it is clear from Eqs. (REF ) and
(REF ), the term \(\delta \Upsilon \)  is responsible for
coupling the inflaton perturbations with those of the radiation
whenever \(c\ne 0\)  and which is known to lead to a growing mode for
the resulting power spectrum [1]}.
